What are the best containers for storing sodium hydroxide (NaOH) and potassium hydroxide (KOH) solutions for soaping? These alkalis are highly reactive and dangerous chemicals, so store them safely.

A sturdy container is important. Lye solution can weigh up to 1 1/2 times more than the same volume of water, so you need a sturdy thick-walled container to properly hold the extra weight. A thin container can easily break, crack, or leak if dropped. A flimsy container can become overly flexible and deform when warmed -- something to keep in mind if you expect to make your lye solution in the container. All things being equal, choose a thick walled, sturdy container over a thinner more flexible one.

A secure leak-proof closure is also important to reduce the chance of leaks or spills. A screw-on cap is safer than a snap-on lid, but even screw-on caps can leak. Check the tightness of the closure by putting plain water in the container, closing the lid, and turning the container upside down to see if the lid leaks.

The container must be made from the right material. Along with a sturdy container fitted with a secure lid, the material of the container must be chemically resistant to the alkali. I recommend certain types of plastics, rather than glass or metal.

Glass. Do not use glass containers for mixing or storing lye solution. NaOH or KOH will etch and weaken glass, leaving the container prone to unexpected breakage. This is true for "Pyrex" glass as well as regular types of glass. Even if the glass is perfectly fine, it will shatter if dropped, unlike metal or plastic, and the shards increase the hazards of cleaning up the mess. More from Irish Lass about glass on the Soapmaking Forum...

Metal. Iron and regular types of steel can trigger rancidity (DOS, dreaded orange spots) in soap and fats. Copper and copper alloys, such as brass and bronze, will also cause rancidity. Aluminum breaks down quickly when exposed to alkali -- it literally dissolves and releases explosive hydrogen gas.

I do not recommend metal containers for mixing or storing lye solution, but metal -- preferably stainless steel -- is always better than glass.

Plastic. The right kind of plastic container is the best choice for mixing and storing lye solution. Some plastics are resistant to alkali, others are not, so it is important to KNOW, not guess, what a plastic container is made of.

The chart below shows the chemical resistance for various plastics. The important line to look at for our purposes is the "Bases/Alkali" line. The green "E" means the plastic has excellent chemical resistance to alkali, so these plastics are the ones to look for when choosing a lye storage container.

Source: http://www.calpaclab.com/chemical-compatibility-charts/

But how can a person translate all those abbreviations? Most of us do not have a clue what ETFE or FLPP is! Thankfully, most plastic containers are marked with a recycle code and/or the abbreviated name of the plastic. The next chart shows how to translate the codes into names.

Of the plastics in this last list, which ones are most easily available to hobby soap makers and are the safest to use with lye solution?

High density polyethylene and polypropylene are best. Most people prefer containers made from high density polyethylene, HDPE, with a #2 recycling code, or polypropylene, PP, with a #5 recycling code. Containers with these codes have excellent resistance to lye up to the maximum 50% concentration. They are heat resistant enough to withstand the heat of a hot lye solution and are often sturdy enough to be a durable, safe container for lye.

Low density polyethylene works, but is not ideal. Low density polyethylene, LDPE, #4 recycling code, has excellent resistance to NaOH or KOH, but this plastic softens somewhat when it is warmed, so it is not ideal for mixing lye solution. Containers made from LDPE often have thin walls and flimsy construction. This means they are not as sturdy and durable as you might want for lye storage, even after the lye solution is cooled to room temperature.

Do not use polyethylene terephthalate, PET or PETG or PETE, recycling code #1. This is the clear plastic often used for disposable soft drink and water bottles. Containers made from PET are often thick and sturdy, seemingly ideal for alkali storage, but this plastic breaks down rapidly when exposed to alkali. The plastic turns progressively cloudy, becomes brittle, and eventually cracks and leaks. NOT SAFE!

Do not use polyvinyl choloride, PVC, #3 recycling code. PVC has excellent resistance to NaOH or KOH. Unfortunately this plastic softens when warm and can deform or stretch. Most PVC containers on the consumer market are also not heavily built enough for long-term safety and durability. NOT SAFE!

Do not use polystyrene, PS, recycling code #6. White foam disposable cups and other inexpensive containers are made of polystyrene. Although polystyrene is chemically resistant to room temperature lye, it is not resistant to hot lye. Polystyrene is brittle and will shatter easily when dropped or bent. NOT SAFE!

Do not use plastics in the "Other" category, recycling code #7. There is no way to know what these plastics are or how chemically resistant and sturdy they might be for storing lye solution. NOT SAFE!